The COALESCE expression is a syntactic shortcut for the CASE expression. That is, the code COALESCE(expression1,...n) is rewritten by the query optimizer as the following CASE expression:

CASE

WHEN (expression1 IS NOT NULL) THEN expression1

WHEN (expression2 IS NOT NULL) THEN expression2

...

ELSE expressionN

END

This means that the input values (expression1, expression2, expressionN, etc.) will be evaluated multiple times. Also, in compliance with the SQL standard, a value expression that contains a subquery is considered non-deterministic and the subquery is evaluated twice. In either case, different results can be returned between the first evaluation and subsequent evaluations.

For example, when the code COALESCE((subquery), 1) is executed, the subquery is evaluated twice. As a result, you can get different results depending on the isolation level of the query. For example, the code can return NULL under the READ COMMITTED isolation level in a multi-user environment. To ensure stable results are returned, use the SNAPSHOT ISOLATION isolation level, or replace COALESE with the ISNULL function. Alternatively, you can rewrite the query to push the subquery into a subselect as shown in the following example.

The ISNULL function and the COALESCE expression have a similar purpose but can behave differently.

Because ISNULL is a function, it is evaluated only once. As described above, the input values for the COALESCE expression can be evaluated multiple times.

Data type determination of the resulting expression is different. ISNULL uses the data type of the first parameter, COALESCE follows the CASE expression rules and returns the data type of value with the highest precedence.

The NULLability of the result expression is different for ISNULL and COALESCE. The ISNULL return value is always considered NOT NULLable (assuming the return value is a non-nullable one) whereas COALESCE with non-null parameters is considered to be NULL. So the expressions ISNULL(NULL, 1) and COALESCE(NULL, 1) although equivalent have different nullability values. This makes a difference if you are using these expressions in computed columns, creating key constraints or making the return value of a scalar UDF deterministic so that it can be indexed as shown in the following example.

In the following example, the wages table includes three columns that contain information about the yearly wages of the employees: the hourly wage, salary, and commission. However, an employee receives only one type of pay. To determine the total amount paid to all employees, use COALESCE to receive only the nonnull value found in hourly_wage, salary, and commission.